Demonstration of Fourier Series in 3D (in fact, it is Discrete Fourier Transform in 3D, FFT). It is easy to understand the connection between time and frequency domain.
All in one 3D picture: time axis, frequency axis, amplitudes, and phases. Everything is interactive - you can drag amplitudes, phases,
draw your own signal, change number of harmonics, turn on sound and listen to the impact of the actions. And more (only in Java version) - you can go to the tutorial
section and watch and listen to a lot of interesting commented automatic demonstrations.
Predefined signals: Sine, cosine, square (rectangle), triangle, sawtooth, beats, sine impulse.
Tutorial experiments: Gibbs phenomenon (Gibbs effect), harmonics and sharpness, discontinuity and sharpness, f0 (fundamental frequency), tone color, DC component, sampling theorem, hearing range, phase and perception, beats, make your own instrument, sine impulse (intro to Fourier transform), vowels (synthetic voice), Shepard tone (infinite music scale).